The present invention relates to the field of electronics, and, more particularly, to analog-to-digital converters and related methods.
The present invention relates to speech processing and voice transmission, for example, during video-conferencing sessions. FIG. 1 diagrammatically illustrates a prior art device for converting an analog signal into a digital signal with automatic gain control. This device includes an analog-to-digital converter ADC receiving at its input the analog signal SA and delivering at its output a digital signal SI sampled at the output sampling frequency Fs. The samples @ of the digital signal SI are delivered successively to an automatic gain control AGC which outputs a digital signal SS. The samples @ of the digital signal SS have an amplitude equal to the product of the amplitude of the input samples multiplied by a gain.
When the amplitude of the analog signal SA increases, and consequently the amplitude of the digital signal SI (i.e., the level of the samples), the automatic gain control AGC will then adjust the value of the gain to restore at its output samples whose levels are equal to the level of the samples before the amplitude jump. FIG. 2 diagrammatically illustrates such an automatic gain control AGC of the prior art which includes a multiplier receiving the samples of the digital signal SI from the converter ADC at the sampling frequency Fs. Furthermore, the multiplier receives the output of a up/down counter CDC whose input is connected to the output of the multiplier by a comparator CMP. The other input of the comparator CMP receives a fixed but programmable threshold TH.
During operation, when the level of the output sample is greater than the threshold TH, the value of the up/down counter CDC (i.e., the value of the gain) decreases at each stage at the working frequency of the up/down counter CDC. The working frequency is much greater than the sampling frequency Fs. On the other hand, if the level of the output sample is below the threshold TH, the value of the up/down counter will increase at each stage.
In response to an abrupt increase in the amplitude level of the analog signal, it is generally required that the amplitude of the digital signal SS delivered by the automatic gain control AGC be returned to its initial value after a predetermined duration Tatt, which varies according to the application. Obtaining the specified value for the duration Tatt is achieved by modulating the duration of operation of the up/down counter within each sampling period Ts.
Depending on the application, the duration Tatt may be short and less than the sampling period Ts corresponding to the sampling frequency Fs. In this case, the automatic gain control AGC will correct the gain several times during the sampling period Ts. Furthermore, during this sampling period the level of the input sample (and consequently the level of the output sample present on the + input of the comparator) will be constant. As a result, convergence of the automatic gain control AGC is not possible. Stated alternatively, the automatic gain control AGC can no longer react properly, as illustrated in FIG. 3.
In the course of the sampling period Ts, at the start of which the digital signal SI delivered by the converter ADC has undergone an amplitude pulse, the level of the output signal SS from the automatic gain control AGC also undergoes an amplitude pulse and then remains constant. During this period, the gain G decreases continuously. Also, when at the start of the next sampling period the next sample of the signal SI is delivered to the input of the automatic gain control AGC, the level of the signal SS drops precipitously below the initial level NI before gradually rising back to the initial level NI in the course of the succeeding sampling periods. This produces a pumping effect on the gain, which in speech processing applications results in audible oscillations.
In view of the foregoing background, it is therefore an object of the present invention to provide a method for converting an analog signal into a digital signal with automatic gain control which overcomes the above problem.
According to the invention, a method for converting an analog signal into a digital signal with automatic gain control includes inputting the analog signal into an analog-to-digital converter of a delta-sigma type having an output sampling frequency. The automatic gain control is performed in the converter downstream of the delta-sigma modulator of the analog-to-digital converter on an intermediate digital signal sampled at an intermediate sampling frequency greater than the output sampling frequency.
Stated alternatively, the invention provides in combination an analog-to-digital converter of a delta-sigma type to perform the analog digital conversion and an automatic gain control in the same converter. This is accomplished by using the digital signals oversampled with respect to the output sampling frequency provided by the delta-sigma converter. Thus, by performing the automatic gain control on intermediate samples delivered at an intermediate sampling frequency higher than the output sampling frequency, it is possible to obtain low durations Tatt which are less than the sampling period of the digital signal obtained (where the sampling period corresponds to the output sampling frequency of the converter) while avoiding the pumping effect of the prior art.
The automatic gain control may be performed at any point downstream of the delta-sigma modulator, particularly at the output of the analog-to-digital delta-sigma modulator. Yet, to obtain improved accuracy it is preferable to perform the automatic gain control on intermediate samples having a relatively large number of bits, providing better resolution. Accordingly, it is advantageous to perform the automatic gain control on the samples of the intermediate signal which are delivered by a comb decimator filter connected to the output of the delta-sigma modulator.
In certain applications, it is necessary to perform a phase of prior calibration of the converter to account for the shift or offset introduced by the delta-sigma modulator. When such a phase of prior calibration is necessary, the invention provides in one embodiment for a constant analog calibration signal to be input into the converter, and for a digital calibration word to be formulated at a location of the converter (the xe2x80x9ccalibration locationxe2x80x9d) based upon the digital samples delivered at the calibration location. The calibration location is situated between the modulator and the location where the automatic gain control is performed. Once this calibration phase has been performed, the digital calibration word is subtracted from each sample delivered to the calibration location and emanating from the analog input signal to be converted. Intermediate digital samples are thereby formed on which the automatic gain control is performed.
In other words, the calibration offset is calculated at a location upstream of the automatic gain control rather than downstream so that the calculation is performed independently of the gain values which may be taken by the automatic gain control. Again, the calibration location may be situated directly at the output of the delta-sigma modulator. Even so, greater accuracy in the calibration offset (digital calibration word) may be obtained by situating the calibration location at the output of the comb decimator filter. During the calibration phase, the formulation of the digital calibration word may include a low-pass filtering of the digital samples delivered to the calibration location. Indeed, this makes it possible to perform rejection of certain noise frequencies which might otherwise disturb the calibration phase.
According to the invention, a device is for analog-to-digital conversion with automatic gain control including an analog-to-digital converter of a delta-sigma type. The analog-to-digital converter includes a delta-sigma modulator, an output decimator filter, and an automatic gain control connected between the modulator and the output decimator filter. The converter may also include a comb decimator filter arranged between the modulator and the output decimator filter. The automatic gain control may then advantageously be arranged between the comb filter and the output decimator filter.
When a calibration is required, the device may include a calibration means or calibrator including a multiplexer having an input connected to the output of the comb decimator filter and first and second outputs. A subtraction means or subtractor is also included having a first input connected to the first output of the multiplexer, a second input, and an output connected to the input of the automatic gain control. Additionally, a memory means or memory (for example, a register) connected between the second output of the multiplexer and the second input of the subtraction means is included. Furthermore, a drive means or driver is included to connect the input of the multiplexer to its second output during a calibration phase to store in the memory means a digital calibration word in response to a constant analog calibration signal present at the input of the converter. The drive means also connects the input of the multiplexer to its first output in a phase of normal operation of the converter.
The calibration means may comprise a low-pass filter connected to the memory means during the calibration phase. The low-pass filter includes, in one embodiment, at least one means or shifter for shifting bits leftward connected between the output of the memory means and the input of the memory means by an adder, and an additional means or shifter for shifting bits leftward connected between the second output of the multiplexer and the adder.